The evolution of wireless communications has resulted in an increase of networks of different technologies and corresponding different air interfaces within an area. As a result, during the course of a single call, a wireless user equipment (UE) may roam among multiple radio access networks (RANs), wherein each such RAN implements a different technology to the other RANs of the multiple RANs, for example, a second generation (2G) and a third generation (3G) circuit switched RAN, such as a GSM (Global System for Mobile communications) network and WCDMA (Wideband Code Division Multiple Access) providing primarily circuit voice service, and a packet data RAN, such as a later generation 3GPP LTE (Third Generation Partnership Project Long Term Evolution) network. As the UE roams among the circuit switched RAN and the packet data RAN providing packet data services, it may be beneficial to system performance to handover the UE between the circuit switched RAN and the packet data RAN for example due to fact that channel conditions associated with the latter RAN may be more favorable than the channel conditions associated with the former RAN. By way of some other examples, an operator of both a legacy circuit network and a packet data network may desire to move the UE from one such network to the other network for purposes of system load balancing and/or for providing certain services to the UE, which cannot be offered in the network into which the UE is presently connected.
Another aspect, in addition to different network technologies within an area, is the management of networks by telecom operators. In a single area there can be one or more networks with different technologies operated by one or more telecom operators. One can directly see that such an environment increases the complexity of a network selection by the user equipment (UE), but it is also inefficient from operators' point of view due to high investment costs of a network or part of it. In order to improve the efficiency the operators may have agreed on sharing at least part of their networks, e.g. in a situation when a network is owned by some companies in a joint ownership basis. As a result, operators may establish an own network for certain areas, but co-utilize a network in some other areas in order to provide extensive communication services to the subscribers.
There exist different models for the network sharing. Mainly, the models relate to the extent of sharing the network and/or part of it. In principle, it is possible to share any part of the network, but basically one can talk about sharing a core network, radio network or both of them. The sharing shall also be understood as covering a share of at least one network element and/or radio resources. More specifically, a network sharing architecture shall, according to at least 3GPP Specification TS 23.251 V11.0.0 (2011-09), allow different core network operators to connect to a shared radio access network. The operators do not only share the radio network elements, but they may also share the radio resources. In addition to this shared radio access network the operators may or may not have additional dedicated radio access networks, like for example, 2G radio access networks. There are two identified architectures to be supported by network sharing in WCDMA. In both of these architectures the radio access network is shared. In addition to that the first architecture, also known as a Gateway Core Network (GWCN) configuration, introduces a solution in which also parts of the core network, such as Mobile Switching Centres (MSC) and Serving GPRS Support Nodes (SGSN), are also shared among different operators. The other network architecture supported by network sharing is the one in which only the radio access network (RAN) is shared and core network nodes operated by different operators are connected to the same Radio Network Controller (RNC). This architecture is known as a Multi-Operator Core Network (MOCN).
The principles of the above described architectures are also applicable to other network technologies than WCDMA. For the Evolved Packet System (EPS) on the packet switched (PS) domain of the architectures is relevant only. For EPS the both above described architectures apply, but the Mobility Management Entity (MME) replaces the SGSN, the eNode B replaces the RNC and the S1 reference point replaces the lu interface between the core network (CN) and UTRAN. For GERAN access only the latter of the architectures (MOCN) described above applies. The RNC is replaced with Base Station Controller (BSC) and lu interface is replaced with A/Gb interface.
An example of a shared network model of MOCN type can be such an architecture in which two core network operators share an UTRAN. However, both operators have their own E-UTRAN and GERAN networks i.e. they are not shared. Thus, the operators share UTRAN RNC and other radio network elements, such as base stations, within an area. It is clear that multiple other network sharing architectures may be configured accordingly.
Next, the network sharing is considered from UE point of view. In order to utilize shared networks the UE must be capable of it. In practice this means that the UE must be able to receive and utilize the additional broadcast system information concerning available core network operators in the shared network. The term ‘core network operator’ shall be understood as an operator that provides services to subscribers as one of multiple serving operators that share at least a radio access network. Each core network operator may provide services to a subscriber of other operators by national or international roaming. Based on the capability of utilizing the additional broadcast system information user equipments can be categorized into two groups. First group are such UEs, which support network sharing in the sense that they are able to select a core network (CN) operator as the serving operator within a shared network on the basis of the additional broadcast system information. Such UEs are called in this context supporting UEs. Correspondingly, second group of UEs i.e. non-supporting UEs do not support network sharing in the sense that they ignore the additional broadcast system information that is specific for network sharing. Worthwhile to mention is that no information concerning the configuration of a shared network is indicated to the non-supporting UE.
Each cell in shared radio access network shall include information concerning available core network operators in a shared network into the broadcast system information. The available core network operators shall be the same for all cells of a Location Area in a shared UTRAN or GERAN network and the core network operators are identified by a PLMN-id, which consists of a Mobile Country Code (MCC) and a two to three digit Mobile Network Code (MNC). Furthermore, each location area of a public land mobile network (PLMN) has its own unique identifier which is known as Location Area Identity (LAI). This internationally unique identifier is used for location updating of mobile subscribers. It is composed of a three decimal digit Mobile Country Code (MCC), a two to three digit Mobile Network Code (MNC) that identifies the PLMN in that country, and a Location Area Code (LAC). Similarly, the available core network operators shall be the same for all cells of a Tracking Area in a shared E-UTRAN network. Each tracking area of a PLMN has its own unique identifier which is known as Tracking Area identity (TAI). The Tracking Area Identity is constructed from the MCC (Mobile Country Code), MNC (Mobile Network Code) and TAC (Tracking Area Code).
When a UE performs an initial access to a shared network, one of available CN operators shall be selected to serve the UE. The selection is dependent on the type of the UE described above. A supporting UE decodes the broadcast system information to determine available core network operators in the shared network and cell (re-)selection procedures. The UE regards both the core network operators indicated in the broadcast system information and conventional networks as individual networks. By the term ‘conventional network’ it is meant a PLMN consisting of radio access network and core network, by which only one serving operator provides services to its subscriber, and subscribers of other operators may receive services by national or international roaming. The core network operators together with all conventional networks are candidate PLMNs in the PLMN-id selection procedure that is performed by the supporting UE. The selection of a core network operator by the UE shall be respected by the network. Supporting UEs inform the radio resource controlling element, such as RNC or eNode B, of the network identity of the chosen core network operator from a list of PLMNs in the broadcast system information. In a UTRAN GWCN configuration, the RNC relays the chosen network identity to the shared core network node. In a MOCN configuration, the RAN routes the UE's initial access to one of the available CN nodes in the shared network. Thus, the supporting UEs inform the RAN of the chosen core network operator, which the RAN uses in order to route correctly.
A non-supporting UE, in its turn, ignores the broadcast system information that is relevant for network sharing. Thus, only the common PLMN together with all conventional networks are candidate PLMNs for the PLMN-id selection procedure that shall be performed by the UE. For indicating a common PLMN i.e. the serving operator for the non-supporting UE a broadcast “common PLMN-id” is used in the PLMN (re)selection processes in UTRAN by the UE. In other words, the common PLMN is a PLMN-id indicated in the system broadcast information as defined for conventional networks (non-shared networks). The non-supporting UEs consider the Common PLMN and the location area code as the Location Area Identity (LAI) and the common PLMN and the location and routing area code as the Routing Area Identity (RAI). The common PLMN does not point out one particular CN operator but any of the CN operators may be able to serve a non-supporting UE. Thus, the RAN, when serving a UE, initially tries to take the UE under control of one CN operator. However, in this initial CN routing phase the RAN has very little information about the subscriber (for example lacks information about the customers IMSI) and for that reason the initial routing phase may not work out. If the routing phase is not successful, a redirection to another CN operator may be required for non-supporting UEs until such an operator is found that can serve the UE. In particularly, for non-supporting UEs of subscribers belonging to one of the CN operators it is not likely to have a roaming agreement with the other CN operators in the shared network, since each CN operator is willing to serve its own subscribers. A special case of non-supporting UEs are inbound roamers with non-supporting UEs, whose home operator has roaming agreement with more than one CN operator in a shared network. Such subscribers may always be redirected to one of the CN operators (e.g. based on International Mobile Subscriber Identity (IMSI) analysis), but it can also be redirected to different operator depending on the geographical area or old Temporary Mobile Subscriber Identity (TMSI), for example.
Moreover, a supporting UE shall use the PLMN-ids that are broadcast in the Multiple PLMN ID List information element. The Common PLMN may or may not be part of the Multiple PLMN ID List. If the common PLMN is part of the multiple PLMN ID list, the common PLMN indicates one of the CN operators for the supporting UEs and no redirection applies. The supporting UEs select one of the Multiple PLMNs and consider the selected PLMN and the location area code as the location Area Identity (LAI) and the selected PLMN and the location and routing area code as the Routing Area Identity (RAI). Thus, in a shared network UEs using the same cell use different location and routing area identities because PLMN identity is different (but location area code is same for all) due to the fact that different CN operators have their own identifier in PLMN-id, and thus in LAI as described above. If the LAI of a cell is not the same as the registered LAI the UEs performs location update, and if RAI of a cell is not the same as the registered RAI the UEs performs routing update.
In order to introduce the problem in a shared network architecture we need to describe a novel feature in EPS network i.e. LTE network. The EPS network consists of Evolved Packet Core (EPC) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
EPS does not currently have any Circuit Switched (CS) domain due to the fact that EPS technology is based on Packet Switched (PS) domain. However, there exist some services, especially at the beginning of EPS deployment that needs to be provided based on CS technology. Such services are e.g. voice calls and short message service (SMS) among others. Yet, a UE capable of EPS cannot utilize e.g. the UTRAN simultaneously while camping in LTE and, as a result, there are no means of originating or terminating any voice calls from e.g. WCDMA CS domain. In order to provide necessary CS based services to subscribers it was decided on a mechanism to support such needs. The mechanism is called Circuit Switched Fallback (CSFB) function, which enables e.g. voice services by using existing WCDMA CS domain functions to be provided to subscribers in EPS.
The CSFB means that the UE is directed from EPS to GERAN (2G) or UTRAN (3G) for voice service. At mobile terminating call a circuit switched call arrives at the MSC Server, the MSC Server sends a page request over the SGs interface to the MME in E-UTRAN. MME starts the paging of the UE in E-UTRAN with the information element CN domain indicator set to CS (Circuit Switch) in the paging message for indicating that the CS network originated the paging. The eNode Bs to be paged are determined from the MME list of TAIs for the UE or from the location information sent in the MSC paging message. When the UE is found i.e. the UE responds with an Extended Service Request for mobile terminating CS fallback to MME, the MME instructs the eNode B to perform CSFB for the UE by sending CSFB indicator in an Initial UE Context Setup message, if UE is in idle mode, or in a UE Context Modification Request message, if UE is in active mode. The eNode B determines into which RAT or carrier frequency the UE should be moved to and directs the UE to retune to a new cell. In case of a PS handover the eNode B determines, preferably based on UE measurements, which cell the UE should be moved to and handover the UE to the new target cell. If the location area identifier of the new cell differs from the one stored in the mobile device, a location update must be performed before the UE responds to the page to establish the call connection. Correspondingly, for mobile originated calls, the UE makes a service request with the Extended Service Request message for mobile originating CS fallback to the MME. The MME requests the eNode B to perform CSFB for the UE by sending CSFB indicator in an Initial UE Context Setup message, if UE is in idle mode, or in a UE Context Modification Request message, if UE is in active mode. The eNode B redirects or handovers the UE to the GERAN (2G) or UTRAN (3G). To determine the target GERAN or UTRAN cell to which the UE should be moved, the eNode B may either solicit measurements from the UE or use its existing information about preconfigured cells. When the target cell has been identified, the eNode B triggers a cell change to the 2G/3G network by sending a Radio Resource Control (RRC) message to the UE. The UE moves to the new cell and performs a radio resource connection using agreed procedures. In addition, before the call originates, a location update may be necessary if the LAI of the new cell differs from the one stored in the UE.
To better understand the CSFB function FIG. 1 illustrates the EPS architecture with the necessary elements from the CS based systems. UE 101 comprises means for interacting with different communications technologies, such as GSM, WCDMA and LTE. Thus, UE 101 is able to be in connection to GERAN 105 over Um interface. Alternatively or in addition, the UE 101 is able to be in connection to UTRAN 106 over Uu interface. Furthermore, the UE 101 comprises means for being interaction with E-UTRAN 102 over LTE-Uu interface. Both the GERAN 105 and UTRAN 106 are controlled by MSC server 104 over either interface A for GERAN 105 or interface lu-cs for UTRAN 106. Interface lu-cs between UTRAN and MSC Servers is especially meant for CS service purposes. Moreover, the E-UTRAN 102 is controlled by MME 103 over S1-MME interface. In order to manage connections and roaming aspects of UEs an interface SGs is configured to enable signaling between MME 103 and MSC 104. Network element SGSN 107 providing support for packet services in GSM and WCDMA systems interacts with MSC server 104 over Gs interface and over Gb interface to GERAN 105 and over lu-ps interface to UTRAN 106.
In order to manage the CSFB there is a need for EPC system to know if the UE supports CSFB. Thus, when the UE attaches to the EPC system using E-UTRAN it indicates that it supports CSFB if that is the case. The indicator makes the MME node inform the MSC Server that a UE supporting CSFB has attached (using the same technique as SGSNs and MSC Servers use for a combined attach). The MME allocates a LAI, which is configured on the MME and may take into account the current TAI (Tracking Area Identity) that the UE is located in. Typically the MME has a mapping table in which the current TAI points out the LAI to be used, and the current TAI and the LAI in the mapping table has more or less the same geographical coverage. Current TAI means the TAI of the current serving cell in E-UTRAN. In this process the UE becomes registered in the current TA (Tracking Area) in EPS (or in a list of TAs) and in one LA (Location Area) in MSC.
If multiple PLMNs are available for the CS domain, the MME performs selection of the PLMN for CS domain based on selected PLMN information received from the eNode B, current TAI, old LAI and operator selection policies on preferred Radio Access Technology (RAT) for CS domain. The PLMN selected for CS should be the same that is used for this UE as a target PLMN for PS handovers or for any other mobility procedures related to CSFB. The selected PLMN-id is included in the LAI which is sent to MSC/VLR and in Attach Accept to the UE.
In order to maintain the location information up-to-date the UE performs a Tracking Area (TA) update when TA update timer expires or when the UE moves into a TA that it is not registered in EPC connection. At each TA update the UE becomes registered in a TA into which the current EPS cell belongs to (or in a list of TAs). If needed, the UE becomes registered in a new LA in MSC, which registration is performed by MME over the SGs interface. In order to be able to perform necessary registrations the MME needs a mapping table between TAI and LAI to be able to contact the right MSC/VLR (Visitor Location Register) and to judge whether LA update is required at a TA update.
The problem arises at least partly from that the UE is moved to an UTRAN cell, or possibly to GERAN cell if Multiple PLMN ID List is introduced in GERAN, in which the UE recognizes another LAI than the one the UE is registered to for the CSFB procedure by as described above. As a result the UE performs a LA update before responding to the paging at mobile terminating call or before performing CS call establishment procedure at a mobile originating call.
The LA update due to different LA is performed when PS handover or RRC CONNECTION RELEASE with redirection indication is used to indicate the need for CSFB to the UE. Since CSFB will approximately double the call setup time compared to normal call setup time due to LA update, it is beneficial to develop such procedures that shorten the CSFB time.
Another aspect is that when the terminal attaches to the EPC system using E-UTRAN it indicates that it supports CSFB, but it does not indicate if it is a supporting UE for UTRAN shared network. The reason for not having a supporting UE indication is that all UEs from a certain version of the technical specifications are supposed to be supporting UEs and the network sharing functionality of understanding multiple PLMNs sent on the broadcast channel is mandatory according to corresponding technical specifications. In reality there exist UEs that do not have the shared network functionality activated e.g. a reason that UE vendors do not activate the functionality because a lack of interoperability tests. Therefore, both the supporting UEs and the non-supporting UEs of the same specification exist in shared networks, but EPC does not know if a UE is supporting or non-supporting terminal. As a result, it does not know if the UE will use common PLMN or one of the multiple PLMNs in a shared UTRAN. During the attach procedure the MME allocates a LAI for a UE, which is configured in the MME. When target network for CSFB, e.g. UTRAN, is shared, the MME has to select a PLMN for this LAI, which is either the common PLMN or a multiple PLMN. However, the selection between common PLMN and the multiple PLMN turns out to be impossible, since the MME cannot be sure that the UE is supporting or non-supporting type of UE. In a case, that the MME allocates the common PLMN and sends the Common PLMN identity to a supporting UE, the UE will have a mismatch of LAI in the target shared network, since this UE will not use the common PLMN but a multiple PLMN. Correspondingly, if the MME allocates a multiple PLMN and sends the multiple PLMN identity to a non-supporting UE, the UE will have a mismatch of LAI in the target shared network, since this UE will not use the multiple PLMN but the common PLMN. The term ‘a multiple PLMN’ refers to one PLMN that is disclosed in the Multiple PLMN ID List broadcast in a location area.
The consequence of both of these alternatives is that the CSFB time is longer due to the required LA update and therefore the call set up time is longer causing dissatisfaction to the user of the terminal.
For in bound roamers with non-supporting UEs and that has roaming agreement with more than one operator, location update may result in that this subscriber is rerouted by RAN to another CN operator than the CN operator whose MSC Server the roamer is registered. The roamer will then be served by another MSC/VLR than the chosen one by the MME. To save the call the network needs to support roaming retry or roaming forwarding for CS fallback between to different CN operators. If roaming retry or roaming forwarding is not supported the CS fallback will fail. The roaming retry takes very long time and is not suitable for CS fallback. Mobile Terminating Roaming Forwarding” procedure and roaming forwarding also adds substantial delay to the CS fallback procedure.
As a summary, it would be advantageous to introduce a solution in which the MME in E-UTRAN is aware of the type of a UE for which it is defining a PLMN during CSFB procedure. This enables the MME to determine the most appropriate PLMN for the UE complying with the allocated LAI.